1. Field of the Invention
The present invention relates to an optical semiconductor device with wavelength selectivity for use as an optical amplifier or laser light source, and a method for amplifying or emitting the light using that device.
2. Related Background Art
Recently, optical communication has been vigorously studied and developed because of its large capacity and the ability of high-speed communication.
To obtain a larger capacity and a higher speed, the technology is transferring to wavelength multiplexing optical communication. Thus, there are needs of a laser light source for emitting light having different wavelengths, and a filter for selecting its wavelength, wherein if the light source is an ordinary semiconductor laser, the variable width of wavelength becomes about 100 .ANG., and if the wavelength multiplexing is made in this range, there are needs of a single mode laser light source having a wavelength spectral band width of 1 .ANG., and a filter for selecting the wavelength with a resolution of 2 to 3 .ANG..
From this point of view, there have been proposed as having the greatest possibility in current application a laser light source of the distributed feedback (DFB) or distributed Bragg reflection type (DBR), and a waveguide filter formed with a distributed reflection portion.
FIG. 1 is a schematic perspective view illustrating one example of a conventional waveguide filter as proposed by Numai et al., in "Tunable wavelength filter using the phase shift control type DFB-LD," Electronic Information Communication Society, Autumn Nationwide Meeting, treatises C-161, 1988.
A filter of FIG. 1 is provided with a grating 82 on a waveguide layer 81, and three divided electrodes 83, 83', 84 formed along a waveguide direction, in which electrodes 83, 83' on both ends have gains, and different refractive indexes varies with the densities of the carrier, respectively, with the distributed reflection wavelength being changed by the grating. A central electrode 84 also controls the refractive index with the variation of carrier density, so that the phase of light passing through a waveguide may be changed to be able to select a wider range of wavelengths.
With the filter as shown in FIG. 1, however, there is a disadvantage that the change of refractive index may be suppressed due to the temperature elevation caused by the current flowing through a waveguide layer. Also, since an active area and a phase adjustment area are provided separately, as shown in FIG. 1, problems may arise, such as the coupling between respective areas and the occurrence of a composite resonator due to reflection at an end face where the active layer is removed. Accordingly, to overcome these problems, disadvantages could not be avoided such that the design of device became complex, and the device would be fabricated with a bad yield.